An a.c. motor does not require a commutator but is not generally regarded as a variable speed machine because its speed is a function of its applied frequency which is normally fixed. If variable frequency and variable voltage power is connected to a commutatorless a.c. motor, it is possible by control of the frequency and amplitude of the applied voltage to provide an efficient variable speed drive. With full rated voltage applied to the stator winding of an induction motor, its speed is nearly proportional to the frequency of the applied voltage and is practically independent of load. By controlling the frequency and amplitude of the stator voltage in such a manner as to maintain constant flux in an induction motor, it is possible to effect efficient variable speed control. With the voltage-to-frequency ratio controlled so as to maintain constant flux, the motor torque is determined by the slip frequency between the applied stator frequency and the output shaft frequency irrespective of motor speed.
The normal steady state operating range of an induction motor is over the portion of the torque/slip characteristic which has a positive slope. However, if the applied frequency and voltage can be controlled so as to keep the absolute slip frequency less than the pullout slip frequency, the machine can be made to operate on the most favorable portion of the torque/slip characteristic under all conditions so it is possible to realize the maximum driving torque of which the machine is capable at all speeds.
Solid state variable frequency and variable voltage power supplies of the cycloconverter type are used for variable speed control of induction motors, but the upper output frequency limit of a cycloconverter is usually less than that of the supply frequency, which places a practical limitation on the range of application for cycloconverter drive systems.
Solid state variable frequency power supplies using an a.c. to d.c. SCR converter followed by a variable frequency inverter are also known for variable speed control of an induction motor, but such systems have relatively low input power factor and relatively high percent of harmonics in the motor current which results in high motor losses and in torque oscillations.
D.c. to a.c. bridge inverter motor drives are known in which switching devices such as transistors connected in a bridge circuit with a d.c. source are selectively driven into conduction at a predetermined rate to convert, or chop the d.c. into a square-wave a.c. output. In these inverters it is customary to insert a filter after the inverter output when a sinusoidal output is desired. The filter extracts the sinusoidal fundamental component from the output which consists of fundamental plus harmonics. The filtering problem is extremely difficult in variable frequency power supplies since conventional filters have a fixed frequency characteristic, and the inverter may have a 20 to 1 frequency range.